The opening will be Ganondorf slashing through an army of hylian soldiers until the King cuts off his hand with the triforce. The king takes the triforce with him and passes it on from generation to generation. Flash forward, huge armies are laying siege to the Castle of Hyrule, Link is sent out to destroy the Trifoce in the fires of Mt. Doom. The title of the game fades in: "The Legend of Zelda: Lord of the Triforce"
I think the beginning of the video resembles the opening of OOT and even seems to use the same animations. The horse-to-warthog fighting seems interesting, will we get a lance?
There weren't many environments shown, I'd guess it isn't really far in development. In fact, with those ground textures I'd say they reused OOT's terrain and threw in a few static meshes. The whole thing is probably far from completion, obviously using lots of placeholders.